Mr Sadr's office said the detained man was a local leader of his political organisation.

The Iraqi prime minister has repeatedly pledged to take action against militias - but he has criticised American raids targeting suspected militia members, saying they could damage national reconciliation efforts.

Moqtada Sadr has in recent years become part of the political process, despite initially opposing it and leading two localised uprisings against US-led forces.

His movement is part of the largest bloc in parliament, the United Iraqi Alliance, alongside the two largest Shia parties. It has strong support among poor Shias, particularly in eastern Baghdad.

Despite repeated accusations that the Mehdi Army, the militia linked to his party, has been involved in death squads carrying out attacks on Sunnis, Mr Sadr has several times called for calm amid rising sectarian tensions.

Analysts say it is likely that he does not have full control of the loose network of militias and armed groups linked to the Mehdi militia.

The development comes ahead of a meeting of Iraqi Sunni and Shia leaders in the holy city of Mecca, Saudi Arabia, which aims to quell the spiralling sectarian violence in Iraq.

Held under the auspices of the Organisation of the Islamic Conference, the gathering of clerics from the two branches of Islam will begin on Friday.

It is expected to back a statement condemning Muslim attacks on other Muslims, supporting the unity of Iraq and calling for the protection of Islamic holy sites.

Iranian role

Separately, the Iranian foreign ministry has responded to comments by the Iraqi president, Jalal Talabani, that Iran and Syria could do more to help stabilise the situation in Iraq.

An Iranian foreign ministry statement said Mr Talabani was not suggesting that Iran was doing anything to destabilise its neighbour.

It said the president meant that Iran could use its spiritual influence over some groups in Iraq to curb violence and create security.

A foreign ministry spokesman said Iran was willing to do its best to help Baghdad because Iraq's security was part of Iran's security.

Iran has offered in the past to talk to the United States about Iraq.

It did not say directly that it was willing to do so, and analysts point out that tensions over Iran's nuclear programme make it difficult to know how Tehran would react to a US offer of talks on Iraq.